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POC++0x atomic implementation in c++98 question about __sync_synchronize()
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  1. POC++0x atomic implementation in c++98 question about __sync_synchronize()
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    copied!<p>I have written the followin atomic template with a view to mimicing the atomic operations which will be available in the upcoming c++0x standard.</p> <p>However, I am not sure that the __sync_synchronize() call I have around the returning of the underlying value are necessary. </p> <p>From my understanding, __sync_synchronize() is a full memory barrier and I'm not sure I need such a costly call when returning the object value.</p> <p>I'm pretty sure it'll be needed around the setting of the value but I could also implement this with the assembly ..</p> <pre><code>__asm__ __volatile__ ( "rep;nop": : :"memory" ); </code></pre> <p>Does anyone know wether I definitely need the synchronize() on return of the object.</p> <p>M.</p> <pre><code>template &lt; typename T &gt; struct atomic { private: volatile T obj; public: atomic( const T &amp; t ) : obj( t ) { } inline operator T() { __sync_synchronize(); // Not sure this is overkill return obj; } inline atomic&lt; T &gt; &amp; operator=( T val ) { __sync_synchronize(); // Not sure if this is overkill obj = val; return *this; } inline T operator++() { return __sync_add_and_fetch( &amp;obj, (T)1 ); } inline T operator++( int ) { return __sync_fetch_and_add( &amp;obj, (T)1 ); } inline T operator+=( T val ) { return __sync_add_and_fetch( &amp;obj, val ); } inline T operator--() { return __sync_sub_and_fetch( &amp;obj, (T)1 ); } inline T operator--( int ) { return __sync_fetch_and_sub( &amp;obj, (T)1 ); } inline T operator-=( T ) { return __sync_sub_and_fetch( &amp;obj, val ); } // Perform an atomic CAS operation // returning the value before the operation inline T exchange( T oldVal, T newVal ) { return __sync_val_compare_and_swap( &amp;obj, oldval, newval ); } }; </code></pre> <p>Update: I want to make sure that the operations are consistent in the face of read/write re-ordering due to compiler optimisations.</p>
 

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